NME4 Loss-of-Function Alters Mitochondria, Triggers Retrograde Signaling and Leads to Cellular Reprogramming

Abstract

NME4 (or nucleoside diphosphate kinase D, NDPK-D, NM23-H4) is a mitochondrial protein with multiple key functions in bioenergetics and signaling. The hexameric NME4 is mainly localized in the intermembrane space, bound to the inner membrane. As we showed earlier, NME4 provides GTP to the mitochondrial GTPase OPA1, stimulates respiration, binds to cardiolipin and promotes its inter-membrane transfer to the mitochondrial surface. Here we deleted either its NDP kinase activity or its cardiolipin interaction and expressed these mutants or wild-type protein in HeLa cells that are almost devoid of endogenous NME4. Both mutations independently led to fragmentation of the mitochondrial network and loss of mitochondrial mass, consistent with disturbed GTP-fueling of the pro-fusion GTPase OPA1. NME4 loss-of-function mutants also showed increased ROS generation and a metabolic shift with reduced cellular respiration and increased aerobic glycolysis. The mitochondrial alterations triggered a specific change in cellular protein expression, with down-regulation of several mitochondrial proteins and altered cell signaling. At the cellular level, NME4 loss-of-function manifested in a morphotypic switch, characterized by inhibited intercellular adhesion, and increased migratory and invasive potential. These data suggest that NME4 loss of function leads to primary alterations in mitochondria that trigger retrograde signaling, finally leading to downstream events responsible for pro-invasive reprogramming of the cell.